1. Field of the Invention
This invention relates to latent hardeners for epoxy resin compositions. In particular it relates to a synergistic combination of two classes of latent hardener. The invention is particularly suited for industrial use, especially in the electronics industry, for example in the manufacture of printed circuit boards and other electronic assemblies.
An uncured epoxy resin to which a latent hardener has been added can be stored in a stable condition at ambient temperature. When the temperature is increased the hardener is activated to cure the resin. A one-part epoxy resin composition of this kind is advantageous but there is a need for such a composition which combines good shelf life with a short curing time at relatively low temperatures.
2. Description of the Related Art
British Patent Specification No. 1,121,196 of Ciba Limited describes a latent hardener for epoxy resins comprising a reaction product of phthalic anhydride and diethylenetriamine. However this hardener requires an elevated temperature of at least 100.degree. C. to cure epoxy resin compositions satisfactorily.
U.S. Pat. No. 4,546,155 Hirose et al assigned to Ajinomoto Co., Inc. describes a latent curing agent for epoxy resins which is an adduct obtained by reacting (i) a polyfunctional epoxy compound, (ii) a compound having at least one OH, NH.sub.2, NH or SH group together with a tertiary amino group in the molecule and (iii) a carboxylic acid anhydride. The compound (b) may suitably be an imidazole compound such as 2-ethyl-4-methylimidazole. The compound (c) may suitably be phthalic anhydride. With hardeners of this type, curing of an epoxy resin composition is possible at temperatures as low as 80.degree. C. but a cure time of at least 30 minutes is required. Such a long cure time is disadvantageous for industrial use.
U.S. Pat. No. 4,546,155 indicates at column 4 lines 55-59 that the latent curing agent of that Patent can be used in combination with a known curing agent such as an acid anhydride, a dicyandiamide, a dibasic acid hydrazide, guanamine, or melamine. However there is no suggestion that any synergistic effect could be obtained.